The exhaust gases generated in an internal combustion engine of a motor vehicle during the operation can be purified by means of a vehicle catalytic converter in order to markedly reduce the pollutant emission of the exhaust gases. Such a purifying of the exhaust gas can be carried out by adding a reducing agent to the exhaust gas of the internal combustion engine in an exhaust gas treatment means. A prior-art method, which is based on such an addition of a reducing agent to the exhaust gas, is the so-called selective catalytic reduction (“SCR”), in which nitrogen oxide compounds are chemically reduced in the exhaust gas.
Such a selective catalytic reduction is especially meaningful in connection with internal combustion engines that generate a high air excess during the combustion of fuel, as this is the case, for example, in diesel engines, in the exhaust gas of which high oxygen concentrations are present. Based on such a high oxygen concentration in the exhaust gas, the use of a conventional three-way catalytic converter for reducing nitrogen oxides is not possible. Furthermore, oxidation catalytic converters, which reduce the emission of CO and CmHn, are frequently used in connection with diesel engines. The catalytically active precious metals, for example, platinum or palladium, can be incorporated in a so-called “wash coat.”
The selective catalytic reduction proper takes place in an area of the exhaust gas treatment device arranged downstream of the diesel oxidation catalytic converter. Ammonia, which must be made available to the exhaust gas treatment device, may be used as a reducing agent. However, ammonia is not injected directly into the exhaust system but is introduced in the form of a reducing agent precursor into the exhaust system to make ammonia available in the exhaust system. Such a precursor may be, for example, a urea-water solution, which is known commercially under the name “Adblue.” The conversion of such a precursor into the reducing agent proper (ammonia) can take place thermally and/or with the use of a catalytic converter within an exhaust gas stream in the area of the internal combustion engine arranged downstream of the exhaust gas treatment means.
The reducing agent can be fed into the exhaust system, for example, by injection by means of an injector. Such an injector may be arranged downstream of the diesel oxidation catalytic converter proper. However, injected droplets of the urea-water solution may reach the surface of the diesel oxidation catalytic converter in certain installation situations and form undesired deposits there. This also applies to the ammonia formed from the urea-water solution, because contact of ammonia with the catalytically active precious metals of the diesel oxidation catalytic converter, for example, in a “wash coat,” may lead to an undesired reaction of the ammonia with platinum or palladium into nitrogen oxides.